The present invention relates to a bidirectional optical transmitting/receiving apparatus, and more particularly to an optical transmitting/receiving apparatus in which a light source for optical transmission and a light receiving element for optical reception are integrated.
One of the most basic optical transmitting/receiving apparatuses for wavelength-division multiplex bidirectional transmission comprises a light emitting unit for converting electric signals into optical signals and sending the converted signals over an optical fiber, and a light receiving unit for receiving optical signals and converting them into electric signals. Also, the apparatuses provide an optical multiplexer/branching filter for combining the two sets of optical signals. However, if each individual unit is configured discretely, the whole system will become very large and, on account of the increased number of connections, its performance will deteriorate.
One of the known examples of conventional optical transmitting/receiving apparatuses reduced in size by integrating a light emitting element, a light receiving element and an optical multiplexer/branching filter is provided by the technique disclosed in the Japanese Patent Laid-open No. 51209 of 1992 (Reference). This optical transmitting/receiving apparatus uses different wavelengths .lambda..sub.1 and .lambda..sub.2 on the transmitting and receiving sides, respectively. The apparatus according to the configuration disclosed in the above-cited literature is smaller than earlier such apparatuses. This is because the light emitting element and the light receiving element are integrated over a substrate on which an optical waveguide and an optical multiplexer/branching filter are formed.
However, as the arrangement of the light emitting element and the light receiving element is not necessarily appropriate in the configuration described in the above-cited Reference. It will be explained in further detail below, the apparatus cannot deliver adequate performance because not only is the size reduction sufficient but also there is a substantial loss of light. Thus, in the configuration described in the Reference, which includes an optical multiplexer/branching filter and a directional coupler, the light of wavelength .lambda..sub.2 incident from the transmission path invites multiplexing/branching losses in two positions. Moreover, since this light returns reflected by a reflective film, it has to travel over an optical waveguide length about double the overall length of the optical waveguide substrate. This further invites a greater optical waveguide loss, making it difficult to receive a light while maintaining a fixed reception level.
On the other hand, the optical fibers needed to be connected to the light emitting element package and the transmission path are arranged on opposite positions with the substrate in-between. For this reason, in a configuration wherein the light emiiting element package and the optical waveguide are to be connected by short optical fibers, V-grooves or the like have to be formed on two sides of the substrate with resultant constraints on the size and ability to realize price reductions of the waveguide substrate.
Furthermore, a reflective film has to be arranged on the side on which a V-groove for laying the short optical fiber for connecting the light emitting element package is to be formed. This gives rise to the additional problem of greater complexity of configuration.